The Urinary System Overview
The Urinary System
Body cells produce wastes as a result of their normal metabolism. For example, when cells break down amino acids, a toxic substance called ammonia (NH3) is formed. In the liver, ammonia combines with carbon dioxide, to form a substance called urea (CH4N2O). While less toxic than ammonia, urea must still be eliminated by the body.
The primary function of the body’s urinary system is to regulate the composition and concentration of many substances found in extracellular fluid, which is all body fluid found outside of cells. One of the major functions of the urinary system is to remove urea. The urinary system does this by filtering out urea found in blood plasma in the kidneys to form a fluid called urine, which is then removed. In addition to urine formation, the kidneys perform other useful functions, as summarized in the table below.
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Organs of the Urinary System
The primary organs of the urinary system are the kidneys, two bean-shaped structures found underneath the ribcage on either side of the spinal cord. In adults, each kidney has a mass of 175 g (6 oz.) and is roughly the size of a fist. Extending from the inward curve of each kidney is a tube called the ureter, which carries urine to the urinary bladder. The urinary bladder is a collapsible sac that temporarily stores urine until it is eliminated from the body. Urine leaves the bladder through another tube called the urethra. The urethra extends through an opening near the vagina in females and through the penis in males.
Bladder
Urethra
Internal Anatomy of the Kidney
As with the other organs of the body, the kidney's structure fits its function. Each kidney consists of two distinct regions, an outer region called the cortex and an inner region called the medulla. The medulla is subdivided into a number of triangular sections called renal pyramids, which are separated by inward extensions of the cortex known as renal columns.
The tip of each renal pyramid ends and empties into a tiny cavity called a minor calyx, which is where urine is first collected. Several minor calyces combine to form a larger cavity called major calyx, and all major calyces funnel into the largest cavity, an L-shaped space called the renal pelvis, which delivers urine formed into the ureter and then to the urinary bladder.
The renal cortex contains more than one million microscopic filtering units called nephrons. The nephron is the basic functional unit of the kidney and is the structure in which urine forms. In its basic form, a nephron consists of a twisting tubule that carries fluid called filtrate, and its surrounding blood vessels, including arteries and arterioles.
Blood Supply of the Kidney
To gain a better understanding of how the nephrons filter blood, it is helpful to understand how blood flows through the kidneys. Unfiltered arterial blood enters the kidneys through the large renal artery. This artery divides into smaller arteries passing deeper into the renal medulla before branching into tiny arcuate arteries (arcuate means bowed) which curve around the border of the cortex and medulla. These arteries then subdivide into numerous microscopic afferent arterioles, which enter the nephrons.
Structures of the Nephron
Recall that the nephron is the basic functional unit of the kidney. Nephrons are responsible for the processes of filtration, reabsorption and secretion, which work together to form urine and will be described in the following section.
Within the nephron, the afferent arterioles deliver blood into a tiny knot of capillaries called the glomerulus. In the glomerulus, dissolved substances in blood are forced out of the capillary and into the hollow walls of the surrounding Bowman’s capsule. The remaining blood then exits the glomerulus via an efferent arteriole. Importantly, the efferent arteriole then forms a web-like network of peritubular capillaries that surround the nephron tubules. These capillaries eventually drain into the renal vein, which carries filtered blood out of the kidney.
The tubular portion of the nephron consists of several parts: the proximal convoluted tubule, the loop of Henle (descending and ascending limbs) and the distal convoluted tubule. The distal convoluted tubule empties into the collecting duct, which carries the filtrate, now called urine, into the renal pelvis.
The Four Steps to Urine Formation
The Four Main Steps to Urine Formation | |
Filtration
Location: glomerulus & Bowman’s capsule | · Filtration is the process of separating waste products from the blood. · In the glomerulus, high blood pressure forces water and small dissolved substances including salts, sugar, amino acids, wastes and urea out of the blood, through the thin capillary walls, and into the Bowman’s capsule. Larger blood components like red blood cells and vital proteins remain in the blood. · This fluid, now called filtrate, flows through the nephron tubules. · The remaining blood then flows from the glomerulus into the efferent arteriole, which then enters the peritubular capillaries that surround the nephron tubules. |
Reabsorption
Location: proximal tubule, loop of Henle, distal tubule | · Reabsorption if the process where the blood reclaims useful substances return from the filtrate, which is refined as it moves through the nephron tubules. · Water and other important substances, such as salts, glucose, and amino acids are reabsorbed from the filtrate back into the blood in the peritubular capillaries. · Reabsorption occurs by two different processes: active transport and osmosis. · Glucose, amino acids and many ions (such as sodium) are reabsorbed by active transport using carrier proteins found in the walls of the renal tubules. · As the concentration of sodium and other ions increases in the peritubular capillaries, an osmotic gradient is created. Water then flows out of the renal tubules to dilute the ions in the blood of the peritubular capillaries. And since sodium and other ions are continually being taken from the filtrate by active transport, roughly 99% of the water in the filtrate is returned to the blood. |
Secretion
Location: distal tubule, collecting duct | · Secretion is the opposite of reabsorption; it occurs when some substances, such as waste products, are actively pushed out of the blood in the peritubular capillaries and into the tubules to be excreted by the body. · In the distal convoluted tubule and collecting duct, certain compounds are moved into the filtrate by active transport. These compounds include urea, creatine, hydrogen ions, ammonia, toxins, and certain drugs such as penicillin. · After the first three steps, the filtrate contained within the nephron is called urine. |
Excretion
Location: collecting duct, renal pelvis, ureters, bladder, urethra | · Excretion is removal of urine from the body. · After processing in the convoluted tubules and loop of Henle, the urine then moves into the collecting duct, which descents into the medulla. Because of the movement of ions during secretion in the previous step, the medulla is saturated with ions. Since the surrounding fluid is hypertonic, water is again drawn out of the collecting ducts by osmosis. This completes the return of water to the blood. · The collecting duct empties into a minor calyx, which further connects to a major calyx and finally, the renal pelvis. Urine then exits the kidney via the ureters, which descend into the urinary bladder, and out of the body through the urethra. |
The kidneys perform their function continuously. They filter the body’s entire blood supply, approximately 5 liters, as many as 400 times each day. With this enormous volume of blood flowing through them on a daily basis, the kidneys process about 180 liters of filtrate daily. Despite all of this filtering, a person excretes only 1.5 liters of urine per day, on average.
Hormonal Control Over Kidney Function
Homeostasis is the body’s stable internal environment. This “steady state” condition is highly important because the body’s cells, tissues and organs can only survive within a narrow range of conditions. The kidneys help maintain homeostasis by controlling blood volume, blood pressure and blood solute concentration, also known as osmolarity.
Blood volume and osmolarity is controlled by a hormone called antidiuretic hormone (ADH), which stimulates the reabsorption of water by the kidneys. Through a complex chemical mechanism, ADH opens membrane pores in the distal convoluted tubule and collecting duct, making them more permeable to water. As a result, more water is reabsorbed and blood volume increases.
The secretion of ADH is stimulated when chemical receptors in a portion of the brain called the hypothalamus detects a rise in sodium and other ions in blood. For example, during dehydration, blood osmolarity increases as there is less water. The hypothalamus triggers the pituitary to release ADH, causing more water to be reabsorbed by the kidneys, which decreases osmolarity. Conversely, when there is an excess amount of water in the body, blood osmolarity decreases as the ions are more diluted. The hypothalamus then inhibits the secretion of ADH, causing less water to be reabsorbed, leaving more in the urine.
Another hormone that regulates kidney function is aldosterone, which is secreted by the adrenal glands; endocrine organs that are found atop of the kidneys. The release of aldosterone is triggered by low levels of sodium ions in the blood. Aldosterone acts on the distal convoluted tubule and has three effects: it stimulates the reabsorption of sodium ions; it causes the reabsorption of water since water molecules “follow” sodium ions by osmosis; and it stimulates the release of potassium ions by tubular secretion. Thus, aldosterone has the net effect of retaining both salt and water, which helps to maintain correct levels of blood volume, pressure, and osmolarity.
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